Sigil was originally created for Planescape as the setting's "home base." According to Steve Winter in 30 Years of Adventure: A Celebration of Dungeons & Dragons, "A movable base, like a vessel of some sort (or an artifact, which was the original idea for the means of traversing the planes) wouldn't do it. It had to be a place that characters could come home to when they needed to, and it had to be central to the nature of the setting."[2] Sigil's fifteen factions were created because, "Vampire: The Masquerade was a particularly hot game at [the] time and one of the ideas in it that we really liked was the clans. Jim Ward wanted to be sure that players had something to identify with and to give them a sense of belonging in this alien venue [Sigil]."[2]

Scott Haring, in his review of the Planescape Campaign Setting for Pyramid, described Sigil as "a strange city with doors to every plane and every reality, and inhabitants from all those planes and realities living together in (more or less) harmony."[3] Trenton Webb of British RPG magazine Arcane calls the city "splendidly bizarre" and declares that "Sigil, The Lady of Pain's citadel, is an elegant gaming construct, yet it can often feel a little hollow", feeling that life in Sigil should be "a swirl of plots, factions and sedition that leaves players' heads spinning, wounds bleeding and experience points tally in overdrive".Trenton Webb reviewed Uncaged: Faces of Sigil for Arcane magazine, rating it a 9 out of 10 overall.[4]

Sigil is first described in the Planescape Campaign Setting boxed set, released in 1994.[5] It is also featured prominently in some later Planescape rulebooks, including In the Cage: A Guide to Sigil (1995),[6]The Factol's Manifesto (1995),[7] and Uncaged: Faces of Sigil (1996),[8] as well as in many adventures, such as The Eternal Boundary (1994),[9]Harbinger House (1995),[10] and Faction War (1998).[11]

A short description of Sigil is in this edition's Dungeon Master's Guide (2003).[12] Information on Sigil can also be found in various 3.0 and 3.5 sourcebooks, such as the Manual of the Planes and the Planar Handbook. A small reference to Sigil is also done in the Epic Level Handbook aside other planar metropolis such as Tu'narath.

Sigil is described in the 4th edition Manual of the Planes[13] and expanded upon in Dungeon Master's Guide 2. The City of Doors, unlike many planes, remains almost completely unchanged from earlier editions.

The main setting [in Planescape], a city "at the center of the multiverse" called Sigil, was run by groups called factions. The factions each had a different outlook on the universe. Thus, by joining one of the factions, or simply by dealing with them as players, you were compelled to think and sometimes even argue about the nature of reality.

Sigil is located atop the Spire in the Outlands.[15] It has the shape of a torus;[15][16] the city itself is located on the inner surface of the ring. There is no sky, simply an all-pervasive light that waxes and wanes to create day and night. Sigil cannot be entered or exited save via portals; although this makes it quite safe from any would-be invader, it also makes it a prison of sorts for those not possessing a portal key. Thus, sometimes Sigil is called "The Cage".[17] Though Sigil is pseudo-geographically located "at the center of the planes" (where it is positioned atop the infinitely tall Spire), scholars argue that this is impossible since the planes are infinite in all dimensions, and therefore there can never truly be a center to any of them, let alone all of them; thus, Sigil is of no special importance. Curiously, from the Outlands one can see Sigil atop the supposedly infinite Spire.

Sigil contains innumerable portals that can lead to anywhere in the Dungeons & Dragons cosmology:[18] any bounded opening (a doorway, an arch, a barrel hoop, a picture frame) could possibly be a portal to another plane, or to another point in Sigil itself. Thus, the city is a paradox: it touches all planes at once, yet ultimately belongs to none; from these characteristics it draws its other name: "The City of Doors".[12]

The ruler of Sigil is the mysterious Lady of Pain.[15] The Lady is sometimes seen in Sigil as a floating, robed Lady with a face bearing a mantle of blades. The Lady does not concern herself with the laws of the city; she typically only interferes when something threatens the stability of Sigil itself. However, she is an entity of inscrutable motives, and often those who cross her path, even accidentally, are flayed to death or teleported to her hidden Mazes, lost forever. It is widely believed that she never speaks, although some unconfirmed (and, most would argue, highly questionable) rumours to the contrary do exist. Sigil is also highly morphic, allowing its leader to alter the city at her whim.[19]

Sigil is, theoretically, a completely neutral ground: no wars are waged there and no armies pass through. Furthermore, no powers (such as deities) are allowed to enter into Sigil; the Lady has barred them from the Cage,[15][20] but some disguised avatars (and Vecna, see below) have made it in and been promptly dispatched by the Lady. It is also of great interest to them, as they could use Sigil to send their worshipers anywhere, and it is at the center of the Outer Planes. Vecna once managed to circumvent the ban by entering while in a transitional state where he wasn't strictly a god when he entered Sigil, and by using Ravenloft as the instrument of his entry, instead of one of Sigil's portals.[21] The Lady has since enacted Wards to prevent this from happening again. Later, it is revealed that the Torus underneath Sigil is the physical manifestation of the Multiverse's Fulcrum, and gods are banned because divine energy disrupts, destabilizes and will eventually "break" it, causing the multiverse to come apart at the seams.[22]Aoskar lived in Sigil for a significantly long time before he was destroyed by the Lady, however. Sigil is hardly peaceful; with such a condensed population, consisting of everything from angelic devas to demonic glabrezu, violence is common, usually befalling the foolhardy, the incautious, or the poor. Most natives of Sigil ("Cagers") are quite jaded as a result of living there.

People coming to Sigil from the Prime Material Plane are often treated as clueless inferiors by the planar elitists who dwell there. They are thus widely referred to as the "Clueless", or more charitably, as "Primes".

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[Planescape] was a setting where cannibal halflings could open an eatery and employ a Drow as the bartender while an angel and a demon fought each other on the floor of the bar.

Sigil is also the setting for the 1999 video game Planescape Torment,[24] in which you play the immortal "Nameless One."[25] In an interview with RPGWatch, Chris Avellone commented on the use of Sigil as the game's main setting, saying "We felt Sigil was the part of Planescape we really had to get right from the outset in case we made more games. It's the signature city, but... we did sacrifice other planar locations so that we could do it."[26]